The present invention relates to electrochromic systems, to electrochiomic monomers and polymers to processes for their preparation, and to the use of the electrochromic systems in devices for variable transparency to electromagnetic radiation.
There are redox-active materials which have different colours in their different oxidation states. This phenomenon is referred to as electrochromicity, and the substances concerned have electrochromic properties. This property can be utilized for modulation of electromagnetic radiation if at least one oxidation state is colourless and at least one other is coloured. Devices in which these properties are utilized are, for example, displays, self-darkening rear-view mirrors in vehicles or dividing screens of variable transparency. They are in principle electrochemical cells.
There are currently 3 different ways of achieving devices of this type:
a) In the solution type, a soluble dye is generated in the electrolyte by an electrochemical redox reaction. The dye molecules formed at one electrode migrate in the field to the counterelectrode, where they are discharged. A medium of this type will always be of low viscosity in order to avoid impairing material transport.
b) If the redox components are selected in such a way that they are in solution in one form, but are insoluble in the other, a precipitate is produced electrochemically at one electrode and re-dissolves on reversal of the current direction.
c) Finally, an electrode coating can be provided at the outset, and this solid layer can be coloured reversibly by the electrochemical reaction.
Method a) is widespread in industry today in a particular variant. In this, a system which consists of the substances RED1 and OX2 dissolved in a solvent is electrolyzed. The substance pair should be colourless in the currentless state of the cell. When a current flows, the substance pair is converted into the OX1/RED2 form, which is as intensely coloured as possible. The two substances are thus selected so that both the oxidation and reduction reactions have complementary colours to one another or are colourless. Thus, one species always becomes the counterelectrode for the other, so that the life of the free-radical ions is very short. When the current is switched off, the cell thus rapidly becomes pale again. A substance pair which is suitable for this process with a complementary counterelectrode has been described by Shelepin et al. (Elektrokhimiya 13, 32-37 (1977); 13, 404-408 (1977); 14, 319-322 (1978)), the industrial use thereof for the modulation of electromagnetic radiation in automobile rear-view mirrors being disclosed in the patent specifications U.S. Pat. No. 4,902,108 (Gentex) and U.S. Pat. No. 5,140,455 (Donnelly).
Method b) uses the high association tendency of free-radical ions of viologens, but these contain absolutely no groups by means of which they can be anchored to surfaces. Only their low solubility allows them to be deposited on the electrodes. These coatings nevertheless have very low cycle stability.
Method c) is less used in industry than widely described. A particular difficulty here has proven to be the achievement of adequate cycle stability. The term xe2x80x9ccycle stabilityxe2x80x9d is taken to mean the frequency with which the sequence of colourless/coloured switching of the cell can be carried out without a change in the absorption spectra or the time behaviour taking place. Even in this case, however, considerable improvements have been achieved through a combination of different substance pairs (WO-A 94/23333, Igen Inc.).
Devices which operate on the principle of soluble dyes are widely used as rear-view mirrors for automobiles (method a). For technical reasons, it is of importance here that the solutions have the lowest possible viscosity, but safety considerations mean that they should have the highest possible viscosity since, in the event of breaking of glass, both the splinters and the cell filling should be held firmly. This requirement is taken into account through the thickening of the solutions by addition of a suitable polymer which increases the viscosity of the solutions, as described, for example, in U.S. Pat. No. 4,902,108. The high viscosity is furthermore desired since the cells, which are generally operated in such a way that the mirror surface is parallel to the earth""s gravity field, tend toward xe2x80x9ccolour separationxe2x80x9d owing to convection. This favours, in particular in the case of large dimensions (truck mirrors), the formation of convections, which can result in separation of the cathodic and anodic products.
However, a considerable problem occurs here in practice, since the viscous solutions can only be introduced into the cells with great difficulty. This is undesired merely from the time expenditure point of view and always means a compromise between fillability and the highest possible viscosity that is desired in the cell. It has therefore repeatedly been proposed, for example in EP-A 0 612 826 (Donnelly) and WO-A 96/03475 (Gentex), to fill the cells with monomers and to carry out the polymerization only when the monomers are in the cell.
The present invention relates to systems which can be operated as desired by principle a) or c) and which are characterized in that at least one of the substituents RED1 and OX2 is a constituent of a soluble, electrochromic polymer.
Polymers which contain OX2 are known as a polymeric viologens (P. M. S. Monk, R. J. Mortimer, D. R. Rosseinsky, xe2x80x9cElectrochromismxe2x80x9d, VCH, 1995) 
Polymers based on acylated 5,10-dihydrophenazines (RED1) have been described in DE-A 4 325 591. 
However, these substances have low solubility and cannot be used for the proposed application. Surprisingly, the polymers according to the invention are readily soluble in organic solvents and do not precipitate even when the charge state is changed.
The present invention relates to an electrochromic system comprising
at least one reversibly electrochemically oxidizable substituent RED1 which is converted into OX1 by electron release at an anode, and
at least one reversibly electrochemically reducible substituent OX2 which is converted into RED2 by electron take-up at a cathode,
where an increase in the absorbance in the visible region of the spectrum from a colourless or weakly coloured form into a coloured form with at least one electron release or electron take-up, in each case the colourless or weakly coloured form is reformed after charge equalization,
characterized in that at least one of the substituents RED1 or OX2 is covalently bonded in a soluble polymer.
The present invention preferably relates to an electrochromic system comprising
at least one reversibly electrochemically oxidizable substituent RED1 which is converted from a colourless or weakly coloured form into a coloured form OX1 by electron release at an anode, with an increase in the absorbance in the visible region of the spectrum, and
at least one reversibly electrochemically reducible substituent OX2 which is converted from a colourless or weakly coloured form into a coloured form RED2 by electron take-up at a cathode, with an increase in the absorbance in the visible region of the spectrum,
where in each case the colourless or weakly coloured form is re-formed after charge equalization,
characterized in that at least one of the substituents RED1 or OX2 is covalently bonded in a soluble polymer.